The hacker gained entry into the network via a SQL injection vulnerability, said Andrew Komarov, CEO of IntelCrawler, a Los Angeles-based security company that tracks hackers and data breaches. The company identified the vulnerability by analyzing w0rm’s screenshot, Komarov said via email.

By gaining entry to the graphics system, w0rm may have also had access to 23 other databases on the same server, Komarov said. But he added it was hard to say if w0rm had data on all of the Journal’s customers. IntelCrawler notified The Wall Street Journal of its findings, he said.

W0rm is known for targeting high-profile companies and also runs an online marketplace, w0rm.in, where hackers can sell information about software flaws.

The hacker “is highly motivated on hacking into big brands and databases with very big number of users,” Komarov said. “Stolen databases are sold by him to various hackers and spammers for spam and malware distribution.”

The same day w0rm tweeted a screenshot of the Wall Street Journal database, he also claimed to have breached the network of publisher Vice.com, again posting a screenshot.

W0rm is believed to have previously gone by the nickname “Rev0lver,” Komarov said. In December 2013, Rev0lver attempted to sell FTP credentials for a server belonging to the broadcaster BBC.

To comment on this article and other PCWorld content, visit our Facebook page or our Twitter feed.